I asked the following earlier today:

I have a 20 year old Lennox Conservator 3... gas furnace/AC combination unit.

The problem started as: When I set the thermostat to "heat" and fan on "auto", the burner would light and stay on for a few minutes then shut down (as it is supposed to do) because the fan was not coming on. If I turned the fan to "on" and set the thermostat to heat, the fan came on and the heat came on... that worked OK in a pinch except the fan didn't shut off when the heat shut off (as it is supposed to do because the fan was in "on", not auto). I have a fan limit switch ordered for Monday, because I thought that was the problem. Today, the problem has changed. Now the fan won't come on at all, in either position. When I put the fan to "on", I hear the relay come on (I assume) and the squirrel cage looks like it is trying to run (barely moving) and you can hear that something is straining to run, but that is all it does. The squirrel cage turns free when I rotate it.

I blew out all the dust that I could get to in the motor, but I don't know what else to check for. The fan limit switch I have is the exact one you have pictured above. Could this be the problem?

Labman: Thanks you so much... you were right on. My fan motor is bad. Hopefully you'll know the answer to the following:

The original motor is a GE, model #5KCP39MGK114BS, 60 hz, 1/2 HP, 1075 RPM 8.2 amp, 1 phase. I can't find this part # anywhere... probably too old. I found one at Grainger, part #4MB38. The mounting is the same, 1/2 HP, 10.2 amp, 1075 RPM, 4 speed. I assume that a higher amp rating is OK. I'm concerned about the 4 speeds... I don't know how many speeds mine has. Any thoughts?

And again... thank you to all who answered... you're awesome.

Most furnaces use 2 fan speeds, although the fan may have more speeds. The installer chooses the proper fan speeds to give the correct temperature rise of the heated air. Too slow and too much heat goes up the chimney. Too fast and the heat is cold and drafty. Your furnace may have the correct temperature rise on it somewhere. The RPM's given likely are the highest speed for each, and should be close enough. Connect the high speed leads to the On relay. Connect one of the lower speeds to the auto relay. See how the heat coming out compares to the old fan. You could post a question to heating and cooling and get better instructions on choosing a fan speed from Northern Heat.

I asked the following earlier today:

I have a 20 year old Lennox Conservator 3...gas furnace/AC combination unit.

The problem started as: When I set the thermostat to "heat" and fan on "auto", the burner would light and stay on for a few minutes then shut down (as it is supposed to do) because the fan was not coming on. If I turned the fan to "on" and set the thermostat to heat, the fan came on and the heat came on...that worked OK in a pinch except the fan didn't shut off when the heat shut off (as it is supposed to do because the fan was in "on", not auto). I have a fan limit switch ordered for Monday, because I thought that was the problem. Today, the problem has changed. Now the fan won't come on at all, in either position. When I put the fan to "on", I hear the relay come on (I assume) and the squirrel cage looks like it is trying to run (barely moving) and you can hear that something is straining to run, but that is all it does. The squirrel cage turns free when I rotate it.

I blew out all the dust that I could get to in the motor, but I don't know what else to check for. The fan limit switch I have is the exact one you have pictured above. Could this be the problem?

Labman: Thanks you so much...you were right on. My fan motor is bad. Hopefully you'll know the answer to the following:

The original motor is a GE, model #5KCP39MGK114BS, 60 hz, 1/2 HP, 1075 RPM 8.2 amp, 1 phase. I can't find this part # anywhere...probably too old. I found one at Grainger, part #4MB38. The mounting is the same, 1/2 HP, 10.2 amp, 1075 RPM, 4 speed. I assume that a higher amp rating is OK. I'm concerned about the 4 speeds...I don't know how many speeds mine has. Any thoughts?

And again...thank you to all who answered....you're awesome.
Hello,

GE 5KCP39MGK114BS can be replaced with the A O Smith 4MB38 motor

I have installed one of the #4MB38 (A O Smith) motors. This motor is most likely made to replace a very large number of applications. This motor has the ears/arms mounted closer to the shaft end that the old GE motor, but this new motor has a longer shaft. This allows it to be used in a number of location.

The A O Smith motor does draw a couple more amps that the GE. This would be using the highest speed.

I installed this motor using the medium and high speeds as this appeared to work out better. I did get a new capacitor as the A O Smith motor called for a 7.5 Mhz verus the GE using a 7.

Both motors have four speeds.
GE
Lo Lo
Lo
Medium
Fast

A O Smith
Lo
Medium
Medium Hi
Fast

Just letting anyone needing this repair know that it will work.

HHH

Need to add something to the above post.

The 4MB38 number is a Graingers part number, the A O Smith part number is 754, if you need to look it up in their catalog or on the web.

HHH